Invasive aspergillosis (IA) is a serious, frequently fatal, infection of immunocompromised patients. The most common pathogens, Aspergillus fumigatus and A. flavus, are common in the environment, but it is not clear if all isolates are equally likely to cause IA. The genomes of both fungi have recently been sequenced, and many genes have been identified as potential contributors to virulence. The long-term objective of this project is to identify genes involved in virulence, trace relationships between environmental populations and clinical isolates, and establish whether mycotoxin production makes an isolate more likely to cause IA. This proposal is unique in that it takes a phylogenetic approach to study virulence in IA. It consists of five interrelated Specific Aims: 1) Establish whether members of all clades of A. fumigatus and A. flavus are equally likely to cause invasive aspergillosis, or whether clinical isolates are restricted to certain clades; 2) Identify genes potentially involved in virulence by comparing phylogenies based on each gene with phylogenies based on neutral markers; 3) Establish whether mycotoxin production is prevalent in isolates of A. fumigatus and A. flavus that cause invasive aspergillosis; and 4) Identify other genes that may be associated with invasive aspergillosis using microarrays; 5) Redirect the Pi's research program to focus on human pathogens and genomics.. The proposed integration of evolutionary biology and information from genomics, applied to a serious pathogen, is consonant with NIH's and NIAID's mission. Relevance: Mortality from IA has decreased thanks to new drugs, but is still >60% in some groups. The main pathogens, A. fumigatus and A. flavus, are opportunistic saprotrophs that are common in the environment. Both produce extremely toxic mycotoxins, though their role in IA is not clearly established. However, aflatoxins, produced by A. flavus and related species, are a serious threat to public health worldwide and recent outbreaks killed hundreds in Africa. Better understanding of which genes contribute to virulence and which environmental populations are likely to cause IA could lead to strategies that may save many lives.